The present invention relates to an apparatus for washing pulp, said apparatus operating in such a way that a substantially continuous layer of pulp is supplied onto a movable, liquid-pervious support surface and washing liquid is supplied to the pulp layer and fed through different areas of the layer in several successive washing stages in such a way that the washing liquid gradually flows in the opposite direction compared to the flow direction of the layer.
The basic structures of the above-type apparatus are disclosed, for example, by U.S. Pat. Nos. 4,266,413, 4,292,123 and 4,769,986. The washing apparatus of these references employs a rotating cylinder surrounded by a stationary shell. The pulp to be washed flows in a substantially continuous web between the casing surface of the cylinder and the shell. The pulp is supplied onto the cylinder at a predetermined inlet point and is discharged from it at an outlet point further on in the rotating direction, while the washing liquid is supplied to the last washing stage close to the outlet of the pulp, from which stage the washing liquid gradually flows through successive washing stages in the direction opposite to the flow direction of the pulp and is discharged to a discharge pipe close to the pulp inlet point.
The basic structures and principles of another apparatus of the above-mentioned type are disclosed, for example, by U.S. Pat. Nos. 4,539,827 and 4,154,644. Both references relate to so called belt washers having a substantially horizontal Fourdrinier type liquid pervious belt as a pulp support surface. Said documents describe a countercurrent washing method where the direction of the flow of the pulp is opposite to the main flow direction of the washing liquid. Both references also disclose washing apparatuses performing several successive washing stages. A characterizing feature of these prior art washers is the manner the washing liquid is fed through the entire system. Both specifications show that the fresh washing liquid is introduced to the pulp layer in the beginning of the last washing stage. The displaced liquid is then collected and fed back to the beginning of the penultimate washing stage. This is continued as long as the displaced liquid from the second washing stage is introduced in the beginning of the first washing stage and the displaced liquid is thereafter removed from the system. In other words, a washing stage receives washing liquid in only one concentration and the displaced liquid is, in principle, collected to one receptacle per washing stage, whereby the concentration of the filtrate is thus equalized.
Several successive washing stages, i.e. several separate displacement operations, are necessary in the washing of pulp as channels are formed in the pulp layer and the washing efficiency of the washing liquid thus falls. In other words, the amount of chemicals and other impurities to be removed from the suspension does not decrease enough during only one displacement stage. Each stage, however, when using ordinary washing equipment, increases the time required for washing the pulp and thus reduces the capacity of the washing plant. This is partly due to the countercurrent washing principle used commonly in washing pulp. It means that fresh washing liquid is introduced to the last washing stage where the suspension contains less chemicals than in the earlier stages. Thus the liquid that has performed a displacement operation once in the last washing stage and been collected after said stage contains such an amount of chemicals that it may be introduced to the previous washing stage i.e. second but last washing stage. The amount of chemicals etc. in the suspension in said washing stage exceeds the one in said washing liquid whereby a successful displacement operation can be performed. This way the same washing liquid can perform three or four (usually) displacements so that the suspension will be washed in said three or four washing stages. It has, however, been found out that the cleanliness of the suspension after said three or four washing stages is not necessarily satisfactory, but one more washing stage should be performed to have the cleanliness to a sufficient level.